1. Field of the Invention
The invention relates to a method for the electrolytic tinning of metal in strip form using an insoluble anode. The metal in strip form is known as tinplate and may be steel not previously tinplated or previously tinned strip.
2. Description of the Prior Art
U.S. Pat. No. 4,181,580 describes a tinning process in which the tinplate in strip form is passed as cathode through a tinning bath containing an acidic liquid electrolyte including tin ions, so that tin ions are deposited on the strip. The electrolyte from the tinning bath is transported to a source of tin ions located outside the tinning bath and is there enriched with tin ions. The electrolyte is then returned to the tinning bath, the concentration of tin ions in the electrolyte in this way being kept up to a desired level. The source of tin ions is a reactor using oxygen in which tin is dissolved by a chemical method. A big disadvantage of the known method is that the unwanted reaction Sn.sup.2+ .fwdarw.Sn.sup.4+ occurs, so that approximately 4% of the tin is converted to sludge. This makes a separate sludge removal system necessary.
Methods of replenishing a plastics electrolyte using electrolytic processes are disclosed in DE-A No. 2027793, GB-A No. 2041408 and FR-A No. 2479856. In particular DE-A No. 2027793 describes a replenishment cell through which the electrolyte, for example a tinplating electrolyte, is passed through a replenishment cell having anode chamber containing a soluble anode which is dissolved to enrich the electrolyte, a cathode chamber and a membrane substantially impermeable to the metal ions of the electrolyte.
Electrolyte replenishment has the advantages that:
(1) Process control is simpler, than in the method of U.S. Pat. No. 4,181,580, since the electrical power used can be controlled much more easily than the feeding of oxygen into the electrolyte. PA0 (2) No or virtually no tetravalent tin ions are formed, and no or virtually no sludge is produced. PA0 (3) The apparatus can be much more compact and cheaper than the oxygen reactor.
However, the known electrolytic replenishment processes have some defects, particularly the need to replace the soluble electrode from time to time. It is also desired to improve efficiency.